1. Field of the Invention
The present invention relates to a method of electrolytically treating a metal plate where the electrolytic reaction is optimally controlled.
2. Description of the Art
Examples of electrolytic treatment on a surface of metal such as aluminum, iron, or the like, include, for example, plating treatment, electrolytically surface-roughing treatment, electrolytically etching treatment, anode oxidation treatment, electrolytically coloring treatment, satin treatment, etc., which are used practically and widely. As a power supply, a suitable direct current, commercial alternating current, superimposed-waveform current, or any other alternating current of a specific or a rectangular waveform controlled by thyristors, etc., is selectively used in accordance with required quality and for the purpose of improvement in reaction efficiency.
FIG. 1 shows a specific example of a conventional system for continuously electrolytically treating a metal web by using graphite electrodes. A metal web 1 is conveyed into an electrolytic cell 4 by a guide roller 2, horizontally conveyed in the electrolytic cell 4 while supported by a support roller 3, and then conveyed out of the electrolytic cell 4 by a guide roller 5. The electrolytic cell 4 is divided into two chambers by an insulator 6. Graphite electrodes 8 and 9, which are main electrodes, are disposed respectively in the two chambers so as to oppose the metal web 1. An electrolyte 28 is stored in a circulation tank 29 and pumped by a pump 30 to electrolyte supply inlets 11 and 12 provided in the electrolytic cell 4. The electrolyte is returned to the circulation tank 29 through a discharge outlet 13, and occupies the gap between the metal web 1 and each of the graphite electrodes 8 and 9. A power source 14 is connected to the graphite electrodes 8 and 9 so as to apply a voltage thereto. In the thus arranged system, the metal web 1 can be continuously subjected to electrolytic treatment.
As the power source 14, a direct current waveform, an alternating waveform, a rectangular alternating waveform, or the like is utilized.
When a metal web is subject to electrolytic treatment, there is a very close relationship between the shape of the treated surface, such as desired pit diameter, pit period, etc., and the electrolytic current conditions. Therefore, the control of an electrolytic current is an important aspect of electrolytic treatment.
When a current of an alternating waveform is used as an electrolytic current, a ratio .gamma. of a forward current mean value I(n) to a reverse current mean value I(r), that is .gamma.=I(n)/I(r), is called a current ratio. In electrolytic treatment, it is known that the shape of a surface treated by electrolytic treatment varies considerably, particularly in accordance with the current ratio. For example, as disclosed in Japanese Patent Post-Examination Publication No. 56-19280, in the electrolytic treatment of an Al plate excellent surface-roughing treatment can be performed so that the Al plate can be an offset printing plate support by using an alternating waveform current supplied so that a voltage in the anode time is larger than a voltage in the cathode time, that is, the current ratio .gamma.=I(n)/I(r)&lt;1.
Conventionally, to control the foregoing current ratio, there have been proposed a method using an alternating current in which the ratio of a forward current mean value to a reverse current mean value is controlled by use of a special power source capable of generating an asymmetrical alternating waveform current, and a method in which a distance between a metal web and each electrode or an effective electrode area is changed.
In the former method, however, there has been a problem in that the power source is complicated thereby increasing the cost thereof or that the distribution of magnetic flux is biased in a transformer. In the latter method, on the other hand, there has been a problem in that the electrolytic treatment cell and electrodes are complicated in structure and therefore the method is not suitable for practical use.